568 research outputs found
Pumping of a Planetesimal Disc by a Rapidly Migrating Planet
We examine the effect of a rapidly migrating protoplanet on a ring of
planetesimals. The eccentricities of the planetesimals are usually increased by
, with the exact increase being proportional to the
protoplanet's mass, and inversely proportional to its migration rate. The
eccentricity distribution is also substantially changed from a Rayleigh
distribution. We discuss the possible implications for further planet
formation, and suggest that the rapid passage of a protoplanet may not prevent
the planetesimal disc from forming further planets.Comment: Five pages, two figures, accepted by MNRA
Numerical simulations of the type III migration:I. Disc model and convergence tests
We investigate the fast (type III) migration regime of high-mass protoplanets
orbiting in protoplanetary disks. This type of migration is dominated by
corotational torques. We study the details of flow structure in the planet's
vicinity, the dependence of migration rate on the adopted disc model, and the
numerical convergence of models (independence of certain numerical parameters
such as gravitational softening). We use two-dimensional hydrodynamical
simulations with adaptive mesh refinement,based on the FLASH code with improved
time-stepping scheme. We perform global disk simulations with sufficient
resolution close to the planet, which is allowed to freely move throughout the
grid. We employ a new type of equation of state in which the gas temperature
depends on both the distance to the star and planet, and a simplified
correction for self-gravity of the circumplanetary gas. We find that the
migration rate in the type III migration regime depends strongly on the gas
dynamics inside the Hill sphere (Roche lobe of the planet) which, in turn, is
sensitive to the aspect ratio of the circumplanetary disc. Furthermore,
corrections due to the gas self-gravity are necessary to reduce numerical
artifacts that act against rapid planet migration. Reliable numerical studies
of Type III migration thus require consideration of both the thermal andthe
self-gravity corrections, as well as a sufficient spatial resolution and the
calculation of disk-planet attraction both inside and outside the Hill sphere.
With this proviso, we find Type III migration to be a robust mode of migration,
astrophysically promising because of a speed much faster than in the previously
studied modes of migration.Comment: 17 pages, 15 figures, submitted to MNRAS. Comments welcom
The evolution of a supermassive binary caused by an accretion disc
The interaction of a massive binary and a non-self-gravitating circumbinary
accretion disc is considered. The shape of the stationary twisted disc produced
by the binary is calculated. It is shown that the inner part of the disc must
lie in the binary orbital plane for any value of viscosity.
When the inner disc midplane is aligned with the binary orbital plane on the
scales of interest and it rotates in the same sense as the binary, the
modification of the disc structure and the rate of decay of the binary orbit,
assumed circular, due to tidal exchange of angular momentum with the disc, are
calculated. It is shown that the modified disc structure is well described by a
self-similar solution of the non-linear diffusion equation governing the
evolution of the disc surface density. The calculated time scale for decay of
the binary orbit is always smaller than the "accretion" time ( is the mass of the secondary component, and is the disc
accretion rate), and is determined by ratio of secondary mass , assumed to
be much smaller than the primary mass, the disc mass inside the initial binary
orbit, and the form of viscosity in the disc.Comment: to be published in MNRA
Quasi-Periodic Formaldehyde Maser Flares in the Massive Protostellar Object IRAS18566+0408
We report results of an extensive observational campaign of the 6 cm
formaldehyde maser in the young massive stellar object IRAS18566+0408
(G37.55+0.20) conducted from 2002 to 2009. Using Arecibo, VLA, and GBT, we
discovered quasi-periodic formaldehyde flares (P ~ 237 days). Based on Arecibo
observations, we also discovered correlated variability between formaldehyde
(H2CO) and methanol (CH3OH) masers. The H2CO and CH3OH masers are not spatially
coincident, as demonstrated by different line velocities and high angular
resolution MERLIN observations. The flares could be caused by variations in the
infrared radiation field, possibly modulated by periodic accretion onto a young
binary system.Comment: 16 pages, 3 figures, accepted for publication in the Astrophysical
Journal Letter
Collisional dust avalanches in debris discs
We quantitatively investigate how collisional avalanches may developin debris
discs as the result of the initial break-up of a planetesimal or comet-like
object, triggering a collisional chain reaction due to outward escaping small
dust grains. We use a specifically developed numerical code that follows both
the spatial distribution of the dust grains and the evolution of their
size-frequency distribution due to collisions. We investigate how strongly
avalanche propagation depends on different parameters (e.g., amount of dust
released in the initial break-up, collisional properties of dust grains and
their distribution in the disc). Our simulations show that avalanches evolve on
timescales of ~1000 years, propagating outwards following a spiral-like
pattern, and that their amplitude exponentially depends on the number density
of dust grains in the system. We estimate a probability for witnessing an
avalanche event as a function of disc densities, for a gas-free case around an
A-type star, and find that features created by avalanche propagation can lead
to observable asymmetries for dusty systems with a beta Pictoris-like dust
content or higher. Characteristic observable features include: (i) a brightness
asymmetry of the two sides for a disc viewed edge-on, and (ii) a one-armed open
spiral or a lumpy structure in the case of face-on orientation. A possible
system in which avalanche-induced structures might have been observed is the
edge-on seen debris disc around HD32297, which displays a strong luminosity
difference between its two sides.Comment: 18 pages, 19 figures; has been accepted for publication in Astronomy
and Astrophysics, section 6. Interstellar and circumstellar matter. The
official date of acceptance is 29/08/200
Survival of icy grains in debris discs. The role of photosputtering
We put theoretical constraints on the presence and survival of icy grains in
debris discs. Particular attention is paid to UV sputtering of water ice, which
has so far not been studied in detail in this context. We present a
photosputtering model based on available experimental and theoretical studies.
We quantitatively estimate the erosion rate of icy and ice-silicate grains,
under the influence of both sublimation and photosputtering, as a function of
grain size, composition and distance from the star. The effect of erosion on
the grain's location is investigated through numerical simulations coupling the
grain size to its dynamical evolution. Our model predicts that photodesorption
efficiently destroy ice in optically thin discs, even far beyond the
sublimation snow line. For the reference case of beta Pictoris, we find that
only > 5mm grains can keep their icy component for the age of the system in the
50-150AU region. When taking into account the collisional reprocessing of
grains, we show that the water ice survival on grains improves (grains down to
~ 20 um might be partially icy). However, estimates of the amount of gas
photosputtering would produce on such a hypothetical population of big icy
grains lead to values for the OI column density that strongly exceed
observational constraints for beta Pic, thus ruling out the presence of a
significant amount of icy grains in this system. Erosion rates and icy grains
survival timescales are also given for a set of 11 other debris disc systems.
We show that, with the possible exception of M stars, photosputtering cannot be
neglected in calculations of icy grain lifetimes.Comment: 12 pages, 9 figures. accepted by A&
Twins Among the Low Mass Spectroscopic Binaries
We report an analysis of twins of spectral types F or later in the 9th
Catalog of Spectroscopic Binaries (SB9). Twins, the components of binaries with
mass ratio within 2% of 1.0, are found among the binaries with primaries of F
and G spectral type. They are most prominent among the binaries with periods
less than 43 days, a cutoff first identified by Lucy. Within the subsample of
binaries with P<43 days, the twins do not differ from the other binaries in
their distributions of periods (median P~7d), masses, or orbital
eccentricities. Combining the mass ratio distribution in the SB9 in the mass
range 0.6 to 0.85 Msun with that measured by Mazeh et al. for binaries in the
Carney-Latham high proper motion survey, we estimate that the frequency of
twins in a large sample of spectroscopic binaries is about 3%. Current
theoretical understanding indicates that accretion of high specific angular
momentum material by a protobinary tends to equalize its masses. We speculate
that the excess of twins is produced in those star forming regions where the
accretion processes were able to proceed to completion for a minority of
protobinaries. This predicts that the components of a young twin may appear to
differ in age and that, in a sample of spectroscopic binaries in a star
formation region, the twins are, on average, older than the binaries with mass
ratios much smaller than 1.Comment: Accepted by the Astronomical Journa
Limiting eccentricity of sub-parsec massive black hole binaries surrounded by self-gravitating gas discs
We study the dynamics of supermassive black hole binaries embedded in
circumbinary gaseous discs, with the SPH code Gadget-2. The sub-parsec binary
(of total mass M and mass ratio q=1/3) has excavated a gap and transfers its
angular momentum to the self--gravitating disc (M_disc=0.2 M). We explore the
changes of the binary eccentricity e, by simulating a sequence of binary models
that differ in the initial eccentricity e_0, only. In initially low-eccentric
binaries, the eccentricity increases with time, while in high-eccentric
binaries e declines, indicating the existence of a limiting eccentricity e_crit
that is found to fall in the interval [0.6,0.8]. We also present an analytical
interpretation for this saturation limit. An important consequence of the
existence of e_crit is the detectability of a significant residual eccentricity
e_LISA} by the proposed gravitational wave detector LISA. It is found that at
the moment of entering the LISA frequency domain e_LISA ~ 10^{-3}-10^{-2}; a
signature of its earlier coupling with the massive circumbinary disc. We also
observe large periodic inflows across the gap, occurring on the binary and disc
dynamical time scales rather than on the viscous time. These periodic changes
in the accretion rate (with amplitudes up to ~100%, depending on the binary
eccentricity) can be considered a fingerprint of eccentric sub-parsec binaries
migrating inside a circumbinary disc.Comment: 10 pages, 7 figures, accepted for publication in MNRA
Gas Accretion from a Circumbinary Disk
A new computational scheme is developed to study gas accretion from a
circumbinary disk. The scheme decomposes the gas velocity into two components
one of which denotes the Keplerian rotation and the other of which does the
deviation from it. This scheme enables us to solve the centrifugal balance of a
gas disk against gravity with better accuracy, since the former inertia force
cancels the gravity. It is applied to circumbinary disk rotating around binary
of which primary and secondary has mass ratio, 1.4:0.95. The gravity is reduced
artificially softened only in small circular regions around the primary and
secondary. The radii are 7% of the binary separation and much smaller than
those in the previous grid based simulations. 7 Models are constructed to study
dependence on the gas temperature and the initial inner radius of the disk. The
gas accretion shows both fast and slow time variations while the binary is
assumed to have a circular orbit. The time variation is due to oscillation of
spiral arms in the circumbinary disk. The masses of primary and secondary disks
increase while oscillating appreciably. The mass accretion rate tends to be
higher for the primary disk although the secondary disk has a higher accretion
rate in certain periods. The primary disk is perturbed intensely by the impact
of gas flow so that the outer part is removed. The secondary disk is quiet in
most of time on the contrary. Both the primary and secondary disks have
traveling spiral waves which transfer angular momentum within them.Comment: 41 pages, 25 PostScript figures, uses AASTeX, to appear in Ap
Modelling Circumbinary Gas Flows in Close T Tauri Binaries
Young close binaries open central gaps in the surrounding circumbinary
accretion disc, but the stellar components may still gain mass from gas
crossing through the gap. It is not well understood how this process operates
and how the stellar components are affected by such inflows. Our main goal is
to investigate how gas accretion takes place and evolves in close T Tauri
binary systems. In particular, we model the accretion flows around two close T
Tauri binaries, V4046 Sgr and DQ Tau, both showing periodic changes in emission
lines, although their orbital characteristics are very different. In order to
derive the density and velocity maps of the circumbinary material, we employ
two-dimensional hydrodynamic simulations with a locally isothermal equation of
state. The flow patterns become quasi-stable after a few orbits in the frame
co-rotating with the system. Gas flows across the circumbinary gap through the
co-rotating Lagrangian points, and local circumstellar discs develop around
both components. Spiral density patterns develop in the circumbinary disc that
transport angular momentum efficiently. Mass is preferentially channelled
towards the primary and its circumstellar disc is more massive than the disc
around the secondary. We also compare the derived density distribution to
observed line profile variability. The line profile variability tracing the gas
flows in the central cavity shows clear similarities with the corresponding
observed line profile variability in V4046 Sgr, but only when the local
circumstellar disc emission was excluded. Closer to the stars normal
magnetospheric accretion may dominate while further out the dynamic accretion
process outlined here dominates. Periodic changes in the accretion rates onto
the stars can explain the outbursts of line emission observed in eccentric
systems such as DQ Tau.Comment: Accepted for publication in MNRA
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